The invention relates to a method for determining the instantaneous winding temperature of the stator winding of a three-phase AC motor, in particular a pole-changeable asynchronous motor.
DE 196 14 900 A1 discloses a method for determining the winding temperature of electrical machines, in particular asynchronous motors. In this reference, the winding temperature is calculated from the motor current and the housing temperature which is measured by a temperature sensor and evaluated by using a neural network. Disadvantageously, this method is relatively complex, in particular the aspect of training the neural network.
The method disclosed in DE 196 14 900 A1 also suggests to calculate the winding temperature from the electrical dissipated energy, with the asynchronous motor being viewed as a system of thermally coupled bodies. The parameters describing the temperature characteristic of the bodies are the thermal resistances between the bodies and the respective heat capacity of the individual bodies. The single input parameter for the calculation is the motor current, which is used to calculate the electrical heat losses which cause heating of the motor. These calculations using this method do not include measuring the housing temperature and are disadvantageously very complex.
It is a therefore an object of the invention to provide a method for determining the winding temperature of the stator winding of three-phase AC motors that have an unfavorable ratio of the active power to the dissipation power and react sensitively to temperature changes and changes in the supply voltage. The heat dissipation power can then be easily determined from the winding temperature of the stator winding.
According to an aspect of the invention, the motor current and a reference temperature are measured continuously at successive times and at a location on the three-phase motor that is thermally connected with the stator, the motor housing or at least a portion of the motor housing, by taking into account the heat capacity of the stator winding and the thermal resistance between the stator winding and the measurement location. The winding temperature is then calculated from values of reference temperature and the motor current. The winding temperature is calculated by calculating sequentially step-by-step the change in the winding temperature for a certain point in time and adding this change to the winding temperature of the immediately preceding point in time. The increase of the winding temperature associated with a certain point in time is set to be proportional to the electrical heat dissipation power that is converted at this point in time into heat in the stator winding, and the decrease of the winding temperature is set to be proportional to the temperature difference between the reference temperature at this point in time and the winding temperature at the immediately preceding point in time. The proportionality factor is defined for the heat dissipation power as the reciprocal value of the heat capacity, whereas the proportionality factor for the temperature difference is the reciprocal value of the thermal resistance multiplied by the heat capacity. The heat dissipation power is calculated from the square of the effective value of the motor current at this point in time multiplied with the temperature-dependent electrical winding resistance of the stator winding. However, the winding resistance for the winding temperature at the immediately preceding point in time in used for this calculation. This produces an iterative calculation method, wherein the change of the winding temperature from one point in time to the next can be calculated mathematically in a very simple manner. Here, the previously calculated winding temperature i.e., the winding temperature at the immediately preceding point in time, is inserted as the instantaneous winding temperature. In other words, the winding temperature is xe2x80x9cfed backxe2x80x9d in the calculation as the instantaneous winding temperature. If the time interval between the consecutive points in time where the motor current and the reference temperature are measured, are selected to be sufficiently small, then the winding temperature can be easily calculated and will agree quite accurately with the instantaneous winding temperature.
Even if an almost arbitrary start value is selected for the winding temperature, the instantaneous winding temperature is derived after only a few seconds, if a period length of the motor current includes a plurality of equidistant points in time.
20 equidistant points are already sufficient for the method to operate accurately within the period length of the motor current.
A change in the number of poles of the three-phase AC motor can be taken into consideration simply by changing the thermal resistance at the same point in time when the number of poles is changed.
Advantageously, the start value for the winding temperature is taken as the reference temperature which is not substantially different from the winding temperature due to the thermal coupling between the stator winding and the housing.
In the simplest case, the change in the winding temperature can be calculated by using the following difference equation:             TW              (        tn        )              -          TW              (                  tn          -          1                )              =            (                        t          n                -                  t                      n            -            1                              )        *          (                                    1            CW                    *                      IMeff                          (              tn              )                        2                    *                      R                          (                              TW                                  (                                      tn                    -                    1                                    )                                            )                                      -                              1                          CW              *              WD                                *                      (                                          TW                                  (                                      tn                    -                    1                                    )                                            -                              TG                                  (                  tn                  )                                                      )                              )      
The accuracy of the method can be improved further by using different values for the thermal resistance when the motor is switched off, since in this case the cooling effect from the motor fan is absent.